In continuous ink jet printing, ink is supplied under pressure to a manifold region that distributes the ink to a plurality of orifices, typically arranged in a linear array(s). The ink discharges from the orifices in filaments which break into droplet streams. The approach for printing with these droplet streams is to selectively charge and deflect certain drops from their normal trajectories. Graphic reproduction is accomplished by selectively charging and deflecting drops from the drop streams and depositing at least some of the drops on a print receiving medium while other of the drops strike a drop catcher device. The continuous stream ink jet printing process is described, for example, in U.S. Pat. Nos. 4,255,754; 4,698,123 and 4,751,517, the disclosures of each of which are totally incorporated herein by reference.
In recent years, the use of the flat face charging scheme described in U.S. Pat. No. 4,636,808 has enabled the spatial resolution of binary array ink jet systems to increase dramatically. This has meant better print quality for high speed text and line drawing applications. In order to improve the quality of pictorial images, however, improvements are needed. Present array ink jet systems print in a binary fashion; in each pixel, they either print a drop of ink or no ink at all. Some pictorial work has been done with binary systems by using various algorithms in which sets of pixels are grouped together to enable the average gray level over the group of pixels to be close to the desired gray level needed in the picture. This process is known in the art as "dithering". In this process, the action of achieving the proper gray level over the group of binary cells detracts from the sharpness (resolution) of the image. A large number of dithering techniques have been described, but all of them group sets of pixels together and result in images which appear "course" or "grainy" to the eye. That is, they trade off graininess and sharpness for the ability to simulate the printing of gray scale images with a binary printing technology.
The image quality of the ink jet array images can be dramatically improved if the color density in each pixel can be varied over a range. Techniques described in U.S. Pat. No. 4,636,808 regarding high spatial density array printing, and U.S. Pat. No. 4,620,196, which deals with printing a variable number of drops on each pixel, are useful for improving image quality. The need for a continuous ink jet system suited to charging arrays of drops at high spatial frequency, utilizing planar charging has been satisfied by U.S. Pat. No. 4,636,808. This patent describes a "flat face" drop charging and deflection scheme for use in a binary printing system. Implicit in the operation of the system is the idea that there are always non-printing drops in the drop stream, and those skilled in the art have observed that these non-printing drops play an important role in developing the electric field which causes drop deflection and selection (see, for example, U.S. Pat. No. 4,613,871.)
It is seen, however, that there is a need for a continuous ink jet system wherein the amount of ink printed in each pixel by the ink jet array can be controlled. The present invention applies techniques used in conventional lithographic and gravure printing to array ink jet systems so that gray scale printing can be dramatically improved.